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  应用层
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  <h1 id="应用层"><a href="#应用层" class="headerlink" title="应用层"></a>应用层</h1><hr>
<h2 id="应用层功能"><a href="#应用层功能" class="headerlink" title="应用层功能"></a>应用层功能</h2><p>在应用进程间传输报文，实现特定类型的数据交互，去执行特定的功能。</p>
<hr>
<h2 id="HTTP-协议"><a href="#HTTP-协议" class="headerlink" title="HTTP 协议"></a>HTTP 协议</h2><p>（超文本传输协议）负责发布和接收 HTML 页面和其他 Web 资源，即 Web 通信。底层通信通过 TCP 协议实现，默认端口号 80 。</p>
<h3 id="资源定位"><a href="#资源定位" class="headerlink" title="资源定位"></a>资源定位</h3><p>HTTP 协议通过 <strong>URL</strong> 来访问资源。URL 即 WEB 内容访问标识符，是一种具体的 URI。不仅唯一标识资源，而且还提供了定位该资源的信息。</p>
<p>通用格式为：<code>协议://IP地址或域名:端口号/文件路径?参数</code> ，如 <a target="_blank" rel="noopener" href="http://localhost:443/student?id=10&amp;name=mrjoker">http://localhost:443/student?id=10&amp;name=mrjoker</a></p>
<h3 id="连接方式"><a href="#连接方式" class="headerlink" title="连接方式"></a>连接方式</h3><ul>
<li><strong>HTTP/1.0</strong> </li>
</ul>
<p>默认使用短连接：访问页面时，客户端和服务器之间每次 HTTP 操作都会单独使用一次 TCP 连接，传输完毕后自动关闭。</p>
<p>客户端和服务器之间都会建立多个 HTTP 会话，开销较大。</p>
<ul>
<li><strong>HTTP/1.1</strong> </li>
</ul>
<p>默认使用长连接：访问页面时，客户端和服务器之间用于传输 HTTP 数据的 TCP 连接不会关闭，直到 HTML 页面调用的所有 web 资源传输完毕。</p>
<p>长连接会在 HTTP 响应头标注：<code>Connection:keep-alive</code>  </p>
<p>但 HTTP/1.1 是串行化处理所有 web 资源请求。一旦有请求无法正常加载，后续请求就会被阻塞。</p>
<ul>
<li><strong>HTTP/2</strong> </li>
</ul>
<p>增加了多路复用功能：让所有数据流共用同一个连接。接收到客户端 HTTP 请求后，服务器会一次性把所需要的 web 资源打包发送。</p>
<ol>
<li>即使有请求无法正常加载，也不会影响处理其它 web 资源请求。</li>
<li>TCP 连接在刚开始发送数据时会限制连接的最大速度，集中发送能更有效地利用 TCP 连接。</li>
</ol>
<h3 id="请求类型"><a href="#请求类型" class="headerlink" title="请求类型"></a>请求类型</h3><p>HTTP 请求有以下四种常用类型：</p>
<ul>
<li><p><strong>GET</strong>：请求服务器数据，请求参数直接附在 URL 上（不够安全，仅适用于简单公开数据的请求和提交）</p>
</li>
<li><p><strong>POST</strong>：向服务器提交数据，数据以表单形式提交（内容长度无上限，且不会被浏览器记录）</p>
</li>
<li><p><strong>PUT</strong>：修改服务器数据（慎用）</p>
</li>
<li><p><strong>DELETE</strong>：删除服务器数据（慎用）</p>
</li>
</ul>
<p>除此之外 HTTP 协议在正式通信前还可能发出 <strong>OPTION</strong> 请求：先询问服务器当前网页所在的域名是否在服务器的许可名单之中，以及可以使用哪些 HTTP 动词和头信息字段。只有得到肯定答复，浏览器才会发出正式的 XMLHttpRequest 请求，否则就报错。</p>
<h3 id="主机识别"><a href="#主机识别" class="headerlink" title="主机识别"></a>主机识别</h3><p>HTTP 协议是无状态协议，本身不保存请求和响应之间的通信状态。</p>
<p>也就是说通信过后服务器将无法相互识别客户端，因此我们必须引入其他方式让服务器记录客户端的信息。</p>
<h4 id="Session-amp-Cookie"><a href="#Session-amp-Cookie" class="headerlink" title="Session &amp; Cookie"></a>Session &amp; Cookie</h4><p>负责记录对方主机信息，在 HTTP 通信中相互识别。cookie 保存在客户端，session 保存在服务器端。</p>
<ol>
<li><p>客户端初次访问服务器时，服务端会自动创建 session 用来标识用户。然后在响应头 Set-Cookie 项向客户端返回 session ID。</p>
</li>
<li><p>客户端再次访问服务器时，会在请求头 Cookie 项向服务器发送 session ID，服务器根据 ID 查询 Session 就可以识别用户。</p>
</li>
</ol>
<p><em>用户和通信的详细信息通常记录在 session 中，cookie 中只保存 session ID ，以避免信息泄露。如果客户端 cookie 被禁用，就需要利用 URL 重写把 session ID 直接附加在 URL 路径上。</em></p>
<h4 id="Token"><a href="#Token" class="headerlink" title="Token"></a>Token</h4><p>token 和 session ID 功能相同，都用来在 HTTP 通信中识别用户，过期刷新：</p>
<ul>
<li>session ID 由服务器随机生成，保存在服务器 session 中。再次访问时只需要直接比对，就可以确认客户端身份。</li>
<li>token 由服务器根据用户 ID 和时间戳经过特定算法生成，服务器不保存。再次访问时服务器需要重新计算并比对，才可以确认客户端身份。</li>
</ul>
<p><em>cookie 本身不安全，浏览器中 token 除放在 cookie 外，还可以放到 localStorage 中存储。浏览器发送请求时会自动携带 session ID，但发送 token 需要手动在代码中设置。由于浏览器加载 image 标签中的地址也会发送 session ID，因此使用 token 可以有效防止 CSRF 攻击。</em></p>
<p>localStorage和sessionStorage的区别;<br>localStorage生命周期是永久，除非用户手动清除，否则这些信息将永远存在。<br>sessionStorage生命周期为当前窗口或标签页，一旦窗口或标签页被永久关闭了，数据也就被清空了。</p>
<hr>
<h2 id="HTTPS-协议"><a href="#HTTPS-协议" class="headerlink" title="HTTPS 协议"></a>HTTPS 协议</h2><p>由于 HTTP 协议使用明文在互联网传送数据，易被不法分子监听和截获。我们又引入了 HTTPS 协议取而代之，默认端口 443。</p>
<p>HTTPS 协议在应用层下方增加了 SSL 层，使用 TLS 协议来加密和解密数据包，这样在互联网上传送的数据将经过加密。</p>
<h3 id="加密方式"><a href="#加密方式" class="headerlink" title="加密方式"></a>加密方式</h3><ol>
<li><p>建立连接后的数据交互采用<strong>对称加密</strong>：加密密钥和解密密钥相同。</p>
</li>
<li><p>建立连接时协商密钥采用<strong>非对称加密</strong>：加密密钥和解密密钥不同，两个密钥互相能解密对方的加密内容。</p>
<p>服务器会公开一个非对称加密密钥（公钥），并保留一个非对称加密码密钥（私钥）。</p>
</li>
</ol>
<h3 id="证书认证"><a href="#证书认证" class="headerlink" title="证书认证"></a>证书认证</h3><p>服务器公钥由数字认证机构 CA 统一认证。CA 会用自己的私钥加密服务器公钥和相应信息，生成数字证书。在客户端向 CA 查询时将证书发送给客户端核对。</p>
<p>CA 根证书（包含公钥）存储在用户的浏览器中，访问网址时会自动比对服务器公钥。</p>
<ol>
<li>客户端向服务器发送信息：请求连接，说明自己支持的加密算法，并给出随机数 A。</li>
<li>服务器向客户端发送信息：同意连接请求，确认合适的加密算法，并给出数字证书和随机数 B。</li>
<li>客户端向 CA 核对数字证书，确认有效后得到服务器公钥。</li>
</ol>
<h3 id="建立连接"><a href="#建立连接" class="headerlink" title="建立连接"></a>建立连接</h3><ol start="4">
<li>客户端向服务器发送公钥加密信息：给出随机数 C。</li>
<li>服务器通过私钥解密信息，对信息 Hash 得到数字签名；然后向客户端发送私钥加密信息：返回数字签名。</li>
<li>客户端通过公钥解密信息，核对数字签名，确认服务器收到随机数 C。</li>
</ol>
<p>之后双方就可以按照约定的对称加密方法，使用三个随机数生成的密钥进行数据交互。</p>
<hr>
<h2 id="HTTP-通信"><a href="#HTTP-通信" class="headerlink" title="HTTP 通信"></a>HTTP 通信</h2><p>Java 程序执行 HTTP 通信的类定义在 java.net 包内，使用时需进行导入。</p>
<p><strong>基本通信方式</strong>：创建连接对象 》设置连接参数和请求属性 》建立连接并发送请求 》读取响应内容 》关闭连接</p>
<h3 id="创建连接对象"><a href="#创建连接对象" class="headerlink" title="创建连接对象"></a>创建连接对象</h3><ul>
<li><p><strong>URL 类</strong>：标记定向资源所在位置，如果格式错误则抛出 <code>IOException</code>。</p>
</li>
<li><p><strong>URLConnection 类</strong>：读取和写入 URL 类定向的资源，在 HTTP 协议中常用 HttpURLConnection 子类。</p>
</li>
</ul>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line"><span class="type">URL</span> <span class="variable">myUrl</span> <span class="operator">=</span> <span class="keyword">new</span> <span class="title class_">URL</span>(<span class="string">&quot;https://www.baidu.com&quot;</span>);</span><br><span class="line"><span class="type">HttpURLConnection</span> <span class="variable">myCon</span> <span class="operator">=</span> (HttpURLConnection)myUrl.openConnetcion();</span><br></pre></td></tr></table></figure>

<h3 id="配置连接"><a href="#配置连接" class="headerlink" title="配置连接"></a>配置连接</h3><p><strong>设置请求参数</strong></p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line">myCon.setRequestMethod(<span class="string">&quot;POST&quot;</span>);         <span class="comment">// 设置连接方法（默认使用GET）</span></span><br><span class="line"></span><br><span class="line">myCon.setDoInput(<span class="literal">true</span>);                 <span class="comment">// 允许进行字符流输入: myCon.getInputStream().read();</span></span><br><span class="line">myCon.setDoOutput(<span class="literal">true</span>);                <span class="comment">// 允许进行字符流输出: myCon.getOutputStream().write();</span></span><br><span class="line"></span><br><span class="line">myCon.setRequestMethod(<span class="number">1000</span>);           <span class="comment">// 设置最长建立连接时间，若超时则抛出 SocketTimeoutException</span></span><br><span class="line">myCon.setRequestMethod(<span class="number">1000</span>);           <span class="comment">// 设置最长数据读取时间，若超时则抛出 SocketTimeoutException</span></span><br></pre></td></tr></table></figure>

<p><strong>设置请求属性</strong></p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// 设置版本</span></span><br><span class="line">myCon.setRequestProperty(<span class="string">&quot;version&quot;</span>, <span class="string">&quot;1.2.3&quot;</span>);            </span><br><span class="line"><span class="comment">// 设置浏览器类型（常用于爬虫伪装）</span></span><br><span class="line">myCon.setRequestProperty(<span class="string">&quot;user-agent&quot;</span>, <span class="string">&quot;Mozilla/5.0 (compatible;  Windows NT 5.1;SV1)&quot;</span>);</span><br><span class="line"><span class="comment">// 设置发送文本类型</span></span><br><span class="line">myCon.setRequestProperty(<span class="string">&quot;Content-Type&quot;</span>, <span class="string">&quot;application/json;charset=utf-8&quot;</span>);</span><br></pre></td></tr></table></figure>



<h3 id="连接并发送请求"><a href="#连接并发送请求" class="headerlink" title="连接并发送请求"></a>连接并发送请求</h3><ul>
<li><strong>connect 方法</strong>：建立连接并发送请求.</li>
<li><strong>getOutputStream 方法</strong>：关闭输出流时自动建立连接并发送输出流请求.</li>
</ul>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line">myCon.connect();</span><br><span class="line"></span><br><span class="line"><span class="type">OutputStreamWriter</span> <span class="variable">out</span> <span class="operator">=</span> <span class="keyword">new</span> <span class="title class_">OutputStreamWriter</span>(myCon.getOutputStream());</span><br><span class="line">out.write(str);                  </span><br><span class="line">out.close();</span><br></pre></td></tr></table></figure>


<h3 id="获取响应数据"><a href="#获取响应数据" class="headerlink" title="获取响应数据"></a>获取响应数据</h3><p><strong>获取响应头信息</strong></p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line">myCon.getResponseCode();        <span class="comment">// 获取响应码（200为连接成功，404为未找到）</span></span><br><span class="line">myCon.getHeaderField();         <span class="comment">// 获取响应头字段</span></span><br><span class="line"></span><br></pre></td></tr></table></figure>

<p><strong>获取响应内容</strong></p>
<ul>
<li><strong>getContent 方法</strong>：获取响应内容。</li>
<li><strong>getInputStream 方法</strong>：输入流获取响应内容，响应表明发送了错误则抛出 <code>IOException</code>。</li>
</ul>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line">myCon.getContent(); </span><br><span class="line"></span><br><span class="line"><span class="type">BufferedReader</span> <span class="variable">in</span> <span class="operator">=</span> <span class="keyword">new</span> <span class="title class_">BufferedReader</span>(<span class="keyword">new</span> <span class="title class_">InputStreamReader</span>(myCon.getInputStream()));</span><br><span class="line"><span class="keyword">while</span> ((str = in.readLine()) != <span class="literal">null</span>) &#123;</span><br><span class="line">    System.out.println(str);</span><br><span class="line">&#125;</span><br><span class="line">in.close();</span><br></pre></td></tr></table></figure>

<h3 id="关闭连接"><a href="#关闭连接" class="headerlink" title="关闭连接"></a>关闭连接</h3><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">myCon.disconnect();</span><br></pre></td></tr></table></figure>

<hr>
<h3 id="HTTP-通信用例"><a href="#HTTP-通信用例" class="headerlink" title="HTTP 通信用例"></a>HTTP 通信用例</h3><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// 网络爬虫</span></span><br><span class="line"><span class="keyword">public</span> <span class="keyword">class</span> <span class="title class_">WebCrawler</span>&#123;</span><br><span class="line">    <span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title function_">getHttpJson</span><span class="params">(<span class="type">int</span> i)</span> &#123;</span><br><span class="line">        <span class="keyword">try</span> &#123;</span><br><span class="line">            <span class="comment">//配置URL</span></span><br><span class="line">            <span class="type">URL</span> <span class="variable">myUrl</span> <span class="operator">=</span> <span class="keyword">new</span> <span class="title class_">URL</span>(<span class="string">&quot;https://static-data.eol.cn/www/school/&quot;</span>+ i + <span class="string">&quot;/info.json&quot;</span>);</span><br><span class="line">            <span class="comment">//配置连接</span></span><br><span class="line">            <span class="type">HttpURLConnection</span> <span class="variable">myCon</span> <span class="operator">=</span> (HttpURLConnection) myUrl.openConnection();</span><br><span class="line">            myCon.setRequestProperty(<span class="string">&quot;user-agent&quot;</span>, <span class="string">&quot;Mozilla/5.0 (compatible; MSIE 6.0; Windows NT 5.1;SV1)&quot;</span>);</span><br><span class="line">            myCon.setConnectTimeout(<span class="number">10000</span>);</span><br><span class="line">            myCon.setReadTimeout(<span class="number">1000</span>);</span><br><span class="line">            <span class="comment">//建立连接</span></span><br><span class="line">            myCon.connect();</span><br><span class="line">            <span class="comment">//如果连接成功</span></span><br><span class="line">            <span class="keyword">if</span> (myCon.getResponseCode() == <span class="number">200</span>) &#123;</span><br><span class="line">                System.out.print(<span class="string">&quot;ID&quot;</span> + i + <span class="string">&quot;的数据读取成功，数据内容：&quot;</span>);</span><br><span class="line">                <span class="comment">//读取返回数据</span></span><br><span class="line">                <span class="type">InputStream</span> <span class="variable">in</span> <span class="operator">=</span> myCon.getInputStream();</span><br><span class="line">                <span class="type">int</span> <span class="variable">cnt</span> <span class="operator">=</span> in.available();</span><br><span class="line">                <span class="type">byte</span>[] b = <span class="keyword">new</span> <span class="title class_">byte</span>[cnt];</span><br><span class="line">                in.read(b);</span><br><span class="line">                <span class="type">String</span> <span class="variable">str</span> <span class="operator">=</span> <span class="keyword">new</span> <span class="title class_">String</span>(b);</span><br><span class="line">                <span class="comment">//输出返回数据</span></span><br><span class="line">                System.out.println(str);</span><br><span class="line">            &#125;<span class="keyword">else</span>&#123;</span><br><span class="line">                System.out.println(<span class="string">&quot;ID&quot;</span> + i + <span class="string">&quot;的数据读取失败，代码：&quot;</span> + myCon.getResponseCode());</span><br><span class="line">            &#125;</span><br><span class="line">        &#125; <span class="keyword">catch</span> (MalformedURLException e) &#123;</span><br><span class="line">            System.out.println(<span class="string">&quot;URL错误，无法查找到资源。&quot;</span>);</span><br><span class="line">        &#125; <span class="keyword">catch</span>(SocketTimeoutException e)&#123;</span><br><span class="line">            System.out.println(<span class="string">&quot;ID&quot;</span> + i + <span class="string">&quot;的数据访问连接超时。&quot;</span>);</span><br><span class="line">        &#125; <span class="keyword">catch</span> (IOException e) &#123;</span><br><span class="line">            System.out.println(<span class="string">&quot;发送的请求存在错误，资源拒绝访问。&quot;</span>);</span><br><span class="line">        &#125;</span><br><span class="line">        <span class="keyword">return</span>;</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>







 
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